As shown in FIG. 12, a known electric motor using this type of magnet embedded rotor has a rotor 11 and a void part 12 inside a stator 10. The rotor 11 comprises four permanent magnets 16 embedded in the form of a square approximately in the iron core 19 of the rotor in which multiple silicon steel plates are laminated and fixed with rivets 18, a rotation axis 13 mounted at the center, and non-magnetic parts 17 at the bilateral edges of each permanent magnet 16 including voids for preventing flux short circuit and others. The parts of the iron core of the rotor facing the outer side of each permanent magnet 16 comprise salient poles 20.
The stator 10 comprises six stator cogs formed at the interval of 60° and three-phase stator winding 15 which winds each stator cog 14.
The magnetic flux density distribution of an electric motor using the rotor 11 with a form as shown in FIG. 12 demonstrates properties close to those of rectangular waves as shown in FIG. 15. Components of higher harmonics are contained with a large quantity, so that the iron loss occurring at the rotation of the electric motor becomes so large that the efficiency decreases.
Thus, to improve this condition, an electric motor with a rotor structure as shown in FIG. 13 is proposed (Patent reference 1).
As shown in FIG. 13, the rotor structure is with the central point of a smaller radius r than the original radius of the iron core 19 of the rotor which is shifted from the central point of the rotor 11 in such a manner that the length of the void part increases gradually over a range from the central part of salient pole 20 to the non-magnetic part 17 on an outer face 24 of the iron core 19 of the rotor. According to the patent reference 1, the magnetic flux density distribution is to show properties close to those of sine waves by reducing the components of higher harmonics to decrease the iron loss, as shown in FIG. 14.
As electric motors in which the torque ripple is made small to decrease vibration/noise, those shown in FIG. 16 are known (Patent reference 2).
Generally, because the iron core 19 of the rotor is not divided between the edge of the permanent magnet 16 and the outer face 24 of the iron core 19 of the rotor, the connection is made with a long and slender bridge part 21. However, in the first embodiment as shown in FIG. 16 (a), an inner face 23 of the non-magnetic part 17 is changed from the chain line's to the solid line's in such a manner that the width of the bridge part 21 becomes narrower gradually over a range from the salient pole side 20 to a reinforced rib part 22 in adjacent two non-magnetic parts 17.
In the second embodiment as shown in FIG. 16 (b), the outer face 24 of the iron core 19 of the rotor is notched as it is from the chain line's to the solid line's to make the width of the notch different.
In the third embodiment as shown in FIG. 16 (c), an inner face 23 of the non-magnetic part 17 is notched in a step-wise manner.
In the fourth embodiment as shown in FIG. 16 (d), the inner face 23 of the non-magnetic part 17 is notched in a polygonal manner.
It is stated that such structures enables to reduce the torque ripple and then vibration and noise.
In electric motors in which the width of the bridge part 21 is changed gradually as the embodiments described above, there was a problem that the magnetic flux of the permanent magnet 16 is dispersed. In the form of the rotor 11 as shown in FIGS. 13 and 16, the non-magnetic parts 17 are mounted at the bilateral edges of the permanent magnet 16. However, a leaked magnetic flux occurs at the adjacent salient pole 20 though the facing stator 10 from the non-magnetic part 17 and this leaked magnetic flux increases the calking torque and consequently increases noise, which was a problem. Thus, it was difficult to realize an induced voltage with a high peak level and less components of higher harmonics.
To solve such problems, the present applicant proposed an electric motor in which a sharp notch 25 is installed to concentrate the magnetic flux density and a protruding portion is formed by extending the bridge 21 between adjacent non-magnetic parts 26 to the outer face 24, as shown in FIG. 5 (Patent reference 3).
Patent reference 1: Japanese Patent Provisional Publication No. 2003-37955
Patent reference 2: Japanese Patent Provisional Publication No. 2000-217287
Patent reference 3: Japanese Patent provisional Publication No. 2005-354798